Photon counting X-ray detectors are known X-ray detectors used in X-ray CT apparatuses. Each of X-ray detecting elements included in a photon counting X-ray detector is capable of counting incident X-ray photons, and outputting a detection signal capable of measuring an energy value (key) of each of the X-ray photons. In the case of using a photon counting X-ray detector, calibration is indispensable to calibrate variations of X-ray energy sensitivity between X-ray detecting elements.